Generally, antennas provided to radar apparatuses receive not only an echo (reflection wave) from a target object or a land, but also an echo from a wave surface (sea surface clutter) and an echo from rain or snow (rain-and-snow clutter). Therefore, in order to suppress (remove) such an unnecessary echo, conventionally, processing of subtracting a predetermined value from a signal level of the echo (gain adjustment processing) has been performed.
Hereinafter, the gain adjustment processing for suppressing the sea surface clutter is exemplary explained with reference to FIGS. 8A to 9C. FIGS. 8A to 8C are charts showing how the sea surface clutter is suppressed by appropriate gain adjustment processing. FIGS. 9A to 9C are views showing display screens when the gain adjustment processing is performed at different levels, respectively.
FIG. 8A is a chart showing echo signals (including the sea surface clutter) before the gain adjustment processing. As shown in FIG. 8A, the sea surface clutter generally has a property in which the signal level thereof is high at a position close to a radar antenna, but it lowers rapidly as it is further distant from the radar antenna. In consideration of this property, there has been a radar apparatus set with a curve for lowering a value of the signal level to be removed (sea surface clutter removing curve). The signal level is lowered more as the sea surface clutter is further distant from the antenna (see FIG. 8B). By subtracting the value obtained from the sea surface clutter removing curve an echo signal having a level indicating only a ship can be extracted from the echo signal before the gain adjustment processing (the sea surface clutter can be suppressed) (see FIG. 8C).
Similarly, by further suppressing other unnecessary echoes, such as the rain-and-snow clutter, a radar image in which the unnecessary echoes are appropriately suppressed (FIG. 9B) can be created. In this manner, necessary echoes can be displayed to be viewed easier comparing to a radar image created without the gain adjustment processing (FIG. 9A).
JP2011-002425A discloses a radar apparatus having a configuration for effectively removing the rain-and-snow clutter. Based on an echo signal received by the radar apparatus, a section determiner provided to the radar apparatus determines whether a distance range where the echo exists is in “a land/target-object section” or “a rain-and-snow/noise section.” Further, the radar apparatus suppresses the rain-and-snow clutter within the distance range determined as “the rain-and-snow/noise section”, by using a value determined according to the echo signal within the distance range.
Note that, the radar apparatus disclosed in JP2011-002425A also has the configuration of displaying the range determined as “the rain-and-snow/noise section” filled in a translucent color.
Meanwhile, it is not easy to set the sea surface clutter removing curve for the gain adjustment processing. For example, when the value of the sea surface clutter removing curve is excessively high as shown in the chart of FIG. 10A, even the echo signal indicating the ship is suppressed (FIG. 10B). As a result, a radar image in which necessary information is also suppressed is created (FIG. 9C).
Therefore, when the radar image is displayed with hardly anything, an operator referring to the radar image may be confused whether the echo indicating the ship is suppressed by the signal processing or the ship itself does not exist. In this case, the operator needs to set the level of the gain adjustment processing to perform the processing again so as to confirm the existence of the ship.
This situation has been a common problem for not only ship radar apparatuses but various kinds of equipments with the possibility of contents detected by a sensor being suppressed through performing signal processing thereon.